Mineralogical Magazine, June 2014, Vol. 78(3), pp. 591–607
Franconite, NaNb2O5(OH)·3H2O: structure determination and the role of H bonding, with comments on the crystal chemistry of franconite-related minerals
M. M. M. HARING* AND A. M. MCDONALD Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada [Received 30 November 2013; Accepted 11 February 2014; Associate Editor: E. Grew]
ABSTRACT
The crystal structure of franconite, NaNb2O5(OH)·3H2O, has been characterized by single-crystal X-ray diffraction using material from Mont Saint-Hilaire, Que´bec, Canada. Results give a = 10.119(2), ˚ b=6.436(1), c = 12.682(2) A and b = 99.91(3)º and confirm the correct space group as P21/c.The crystal structure, refined to R=4.63% and wR2 =11.95%, contains one Na site, two distorted octahedral Nb sites and nine O sites. It consists of clusters of four edge-sharing Nb(O,OH)6 octahedra, linked through shared corners to adjacent clusters, forming layers of Nb(O,OH)6 octahedra. These alternate along [100] with layers composed of NaO(H2O)4 polyhedra, the two being linked together by well defined H bonding. The predominance of H bonding, essential to the mineral, results in a perfect {100} cleavage. Chemical analyses (n = 7) of four crystals give the empirical formula (Na0.73Ca0.13&0.14)S=1.00(Nb1.96Ti0.02Si0.02Al0.01)S=2.01O5(OH)·3H2O (based on nine oxygens) or ideally NaNb2O5(OH)·3H2O. Franconite is crystallo-chemically related to SOMS [Sandia Octahedral Molecular Sieves; Na2Nb2 xMxO6 x(OH)x·H2O with M = Ti, Zr, Hf], a group of synthetic compounds with strong ion-exchange capabilities. Both hochelagaite (CaNb4O11·nH2O) and ternovite (MgNb4O11·nH2O) have X-ray powder diffraction patterns and cation ratios similar to those of franconite indicating that these minerals probably have similar structures.
KEYWORDS: franconite, hochelagaite, ternovite, crystal structure, Mont Saint-Hilaire, hydrogen bonding.
Introduction 60.001 mm; Jambor et al., 1986). It has also FRANCONITE,NaNb2O5(OH)·3H2O, is the Na- been discovered in other agpaitic environments dominant endmember of a ternary system with including Mont Saint-Hilaire (Horva´th and Gault, other hydrated Nb oxides including hochelagaite 1990), the Saint-Amable sill (Horva´th et al., (CaNb4O11·nH2O; Jambor et al., 1986) and 1998), the Khibiny massif (Pekov and Podlesnyi, ternovite (MgNb4O11·nH2O; Subbotin et al., 2004), the Vuoriyarvi alkaline-ultrabasic massif 1997). The mineral was first discovered at the (Belovitskaya and Pekov, 2004) and the Francon quarry in the St-Michel district of Vishnevogorsk alkali complex (Nikandrov, Montreal, Que´bec where it occurs in vugs 1990). In the original description, Jambor et al. within dawsonite-bearing sills as white polycrys- (1984) gave a unit cell of a = 22.22(1), b = talline globules (average diameter ~0.15 mm) 12.857(5), c = 6.359(4) A˚ and b = 92.24(6)º. Data consisting of radiating, extremely thin, blades from chemical analyses indicated the ideal (average dimensions = 0.030 mm60.005 mm formula Na2Nb4O11·nH2O(n =8 9) but with variable Na:Ca ratios, as well as trace Ti, Al, Si and Sr (Jambor et al., 1984). It is quite likely that * E-mail: [email protected] the variable Na:Ca ratios are a function of both DOI: 10.1180/minmag.2014.078.3.09 franconite and hochelagaite being found together
# 2014 The Mineralogical Society M. M. M. HARING AND A. M. McDONALD
in the same complex globules, an observation Monte´re´gie (formerly Rouville County), Que´bec, made during this current study. The two minerals Canada, where it occurs as a rare mineral in also have similar optical properties, making them sodalite-syenite, miarolitic cavities and hornfels indistinguishable megascopically (Jambor et al., microenvironments (Horvath and Gault, 1990). 1984, 1986). The material used in this study came from To date, only X-ray powder diffraction (XRPD) sodalite-syenite where it occurs in association studies have been completed on franconite as the with microcline, analcime, sodalite, siderite and small crystal size of the mineral has precluded pyrite (Table 1). Franconite occurs in spherical analysis by typical single-crystal methods. As aggregates of radiating bladed crystals that are such, the true chemical formula and crystal ~0.15 0.20 mm in diameter. Individual crystals structure of the mineral have remained largely of franconite are, on average, 0.008 mm6 unknown. With the introduction of extremely 0.020 mm60.060 mm, euhedral, with a perfect bright X-ray sources arising from a combination {100} cleavage and they display the forms of rotating-anode generators coupled with multi- pinacoid {100} and {011}. layer optics, incident-beam paths and highly sensitive detectors, it is now possible to study Chemistry the crystal structures of minerals (Cooper and Hawthorne, 2012) such as franconite. In this study Chemical analyses of franconite were performed results are reported from a successful single- with a JEOL JSM 6400 scanning electron crystal study of franconite, a description of its microscope using a voltage of 20 kV, a beam structure is presented and the complex nature of H current of ~1 nA and a beam width of 1 mm. Data bonding within franconite is described, along with from energy-dispersive spectrometry (EDS) were comments on the significance of the crystal collected using the following standards (X-ray structure in the context of the chemical variation lines): CaTiO3 (CaKa,TiKa), synthetic MnNb2O6 in franconite and the related minerals: hochela- (NbKa) and albite (NaKa,SiKa,AlKa). The gaite and ternovite. As franconite is a late-stage globules analyzed in this study were found to mineral occurring in many agpaitic environments, contain crystals of near endmember franconite, understanding its crystal structure can help to along with compositions intermediate between better understand processes related to the low- franconite and hochelagaite (i.e. with variable temperature alteration of Nb-rich minerals. Na:Ca ratios) and near endmember hochelagaite. None of the crystals appears to be chemically zoned when viewed perpendicular to the layering. Occurrence and description Due to the variation of Na and Ca in the globules The franconite used in this study was collected at of franconite, 15 separate crystals were isolated the Poudrette quarry, La Valle´e-du-Richelieu, for chemical analysis. Of these, four were found
TABLE 1. Modal abundances and descriptions of minerals associated with franconite.
Mineral Modal abundance (%) Description
Microcline Light grey subhedral crystals ranging from ~2 to 5 mm in ~45 (KAlSi3O8) diameter. Analcime Translucent, anhedral to subhedral crystals ranging from ~25 Na2(Al2Si4O12)·2H2O 2 to 3 mm in diameter. Sodalite Light blue anhedral crystals ranging from 1 to 3 mm in ~15 Na8(Al6Si6O24)Cl2 diameter. Siderite Light brown to tan euhedral crystals exhibiting the form FeCO3 ~10 rhombohedron {101¯1} and ranging from 3 to 5 mm in diameter Pyrite ~5 Anhedral crystals ranging from 1 to 3 mm in diameter FeS2
592 CRYSTAL CHEMISTRY OF FRANCONITE to have franconite-rich compositions, with the (Babechuk and Kamber 2011). Both Nb and Ta remaining ones having either hochelagaite-rich or share crystal-chemical similarities such as iden- intermediate franconite–hochelagaite composi- tical charges (3+, 5+) as well as similar ionic radii tions. Chemical analyses (n = 7) of the four (~0.64 A˚ for [6]Nb5+ and [6]Ta5+; Shannon et al. franconite-rich crystals give the average (range) 1976), so would be predicted to substitute readily composition (wt.%): Na2O6.45(5.03 8.87), for one another in crystal structures. Niobium- CaO 2.00 (0.34 3.27), Nb2 O 5 74.23 bearing minerals such as vuonnemite 4+ (72.50 75.43), TiO2 0.52 (0.40 0.75), SiO2 [Na11Ti Nb2(Si2O7)2(PO4)2O3(F,OH); Ercit et 4+ 0.42 (0.27 0.81), Al2O3 0.11 (0.04 0.27) and al., 1998], epistolite [Na4Nb2Ti (Si2O7)2 H2O (calc.) 17.96 and total = 101.69, corre- O2(OH)2(H2O)4; Sokolova and Hawthorne, sponding to the empirical formula: 2004] and laurentianite {[NbO(H2O)]3(Si2O7)2 (Na0.73Ca0.13& 0.14) S =1.00(Nb1.96Ti0.02 [Na(H2O)2]3; Haring et al., 2012} from alkaline Si0.02Al0.01)S=2.01O5(OH)·3H2O(basedon environments similar to those in which franconite 9 oxygens) or ideally NaNb2O5(OH)·3H2O. forms, also exhibit a virtual absence of Ta. These Although the Na site was determined to be fully observations may suggest a possible fractionation occupied in the crystal-structure refinement, the between Nb and Ta occurring prior to the empirical formula indicates a minor deficiency in formation of these minerals, which would also the Na site, a feature that may be explained by the hold for franconite. The exact cause of the Nb-Ta instability of the mineral and possible migration decoupling mechanism remains unknown, but of Na under the electron beam. The ideal formula given that it is observed in Nb-bearing minerals originally proposed for franconite by Jambor et al. from a number of agpaitic environments, the (1984) was Na2Nb4O11·nH2O, determined on the implication is that it must be related to the basis of SNb+Ti+Si+Al = 4, as the nature of the geochemical processes leading to the formation of anions was unclear. The ideal formula proposed these unusual lithologies. here is derived from results of the crystal- structure analysis and to a greater extent, more Raman and infrared spectroscopy clearly defines the content and roles of (OH) and H2O in the mineral. In light of the obvious Raman and infrared spectroscopy of franconite chemical variation that may exist in franconite, was carried out to investigate the presence of OH data from wavelength-dispersive spectrometry and H2O. The Raman spectrum of franconite was (WDS) were also collected on the crystal used collected using a Horiba Jobin Yvon XPLORA for the single-crystal X-ray structure study. These Raman spectrometer interfaced with an Olympus were obtained using a Cameca SX-100 electron BX41 microscope over a range of 50 4000 cm 1. microprobe operated with an excitation voltage of The spectrum (Fig. 1) represents an average of 15 kV, 5 nA current and a 15 mm beam width three spectra, each collected with 20 s acquisition (chosen to reduce migration of Na). The following cycles. An excitation radiation of l = 638 nm was standards (X-ray lines) were used: albite (NaKa), used, along with a 600 grating and 1006 olivine (MgKa), diopside (CaKa,SiKa), andalu- magnification (producing a beam of diameter = site (AlKa), titanite (TiKa), Ba2NaNb5O15 2 mm). The excitation radiation of l = 638 nm was (NbKa) and SrTiO3 (SrKa). The compositions chosen to eliminate fluorescence peaks in the of two points taken from the crystal used for region of ~2000 2500 cm 1 which were single-crystal analysis are similar to those observed when spectra were collected using l = presented above, confirming the data obtained 532 nm. Calibration was undertaken using the by energy-dispersive spectrometry. Data from the 521 cm 1 line of a Si wafer. The Raman spectrum two data points obtained by WDS give the of franconite contains bands in two regions: at 1 average composition: Na2O7.60,CaO0.35, ~212 924 and 3416 cm (Fig. 1). The first Nb2O5 75.05, TiO2 0.61, SiO2 0.02, Al2O3 0.03 consists of sharp, moderate to high intensity peaks and H2O (calc.) 16.31 total 100%. at 212, 297, 391, 461, 583, 661, 879 and 1 Other elements were sought but were not found 924 cm. Those peaks occurring below to be present in statistically meaningful concen- 500 cm 1 are associated with Na O/Ca O trations. In particular, data from EDS and WDS bonding (Table 2). The remaining high-intensity indicate franconite is devoid of Ta, an interesting peaks are attributed to the two distinct distorted observation considering the strong geochemical octahedral sites associated with Nb (see below). coupling behaviour between Nb and Ta Those peaks attributed to Nb O were assigned
593 M. M. M. HARING AND A. M. McDONALD
FIG. 1. Raman spectrum of franconite.
based on data on Nb oxides presented by Jehng Na2Nb2 xMxO6 x(OH)x·H2O with M = Ti, Zr, and Wachs (1990). Peaks at 583 and 661 cm 1 are Hf; Nyman et al., 2002; Iliev et al., 2003], are attributed to symmetric stretching of the similar to that of franconite. The spectra of the Nb O Nb linkages associated with two crystal- synthetic phases both have a single, moderately lographically independent Nb(O,OH)6 octahedra strong Raman band in the region of (Fig. 1). The peaks at 879 and 924 cm 1 are ~750 800 cm 1, associated with symmetric attributed to symmetric stretching of possible stretching of Nb=O bonds, as well as two Nb=O bonds present in the distorted Nb(O,OH)6 moderately strong Raman bands in the region of sites (Jehng and Wachs, 1990). The final Raman 450 650 cm 1, associated with Nb O Nb band is a single, broad, low intensity peak at linkages from two crystallographically indepen- 1 3416 cm associated with O H stretching dent NbO6 sites (Jehng and Wachs, 1990). In the (Williams, 1995). The Raman spectra of the region 1200 1700 cm 1, bands associated with synthetic phases HCa2Nb3O10 and KCa2Nb3O10 H O H stretching could not be detected using (Jehng and Wachs, 1990) as well as those of Raman spectroscopy, probably because of the SOMS [Sandia Octahedral Molecular Sieves; weak Raman scattering abilities of water.
TABLE 2. Observed absorption bands and band assignments for the Raman spectrum of franconite.
Raman absorption band (cm 1) Suggested assignment
3416 O H stretching 924 Symmetric stretching of Nb=O double bond 879 Symmetric stretching of Nb=O double bond 661 Nb O Nb linkages – symmetric stretching 583 Nb O Nb linkages – symmetric stretching 461 Na O/Ca O 391 Na O/Ca O 297 Na O/Ca O 212 Na O/Ca O
594 CRYSTAL CHEMISTRY OF FRANCONITE
FIG. 2. FTIR spectrum of franconite with transmittance peaks indicated.
To confirm the presence of water in franconite spectra of franconite are identical. The first two and in light of the fact that water is a weak Raman regions at ~677 cm 1 and 874 1028 cm 1 are scatterer but a strong absorber of infrared similar to those in the Raman spectrum at 563, radiation, an infrared (FTIR) spectrum over the 646, 815 and 877 cm 1; these are attributed to the range 600 4000 cm 1 was collected using a symmetric stretching of Nb O Nb and vibra- Bruker Alpha spectrometer equipped with a KBr tions of terminal Nb=O bonds, respectively beam splitter and a DTGS detector. This spectrum (Table 3; Fielicke et al., 2003). The third (Fig. 2), obtained by averaging 128 scans with a region, centered at ~1658 cm 1, consists of a resolution of 4 cm 1, reveals four distinct bands strong band at ~1658 cm 1 with a shoulder at at ~677 cm 1, 874 1028 cm 1, 1658 cm 1 and ~1617 cm 1 and is attributed to H O H bending 3204 3422 cm 1. Overall, the spectrum is (Williams, 1995). The fourth region, at similar to that reported by Jambor et al. (1984) ~3204 3422 cm 1, is consistent with the weak and, with the exception of an additional band in Raman band at 3303 cm 1 and consists of a the region of 1658 cm 1, the FTIR and Raman strong band at ~3356 cm 1 with shoulders at
TABLE 3. Observed transmittance bands and band assignments for the FTIR spectrum of franconite.
FTIR transmittance band (cm 1) Suggested assignment
3422 O H stretching 3356 O H stretching 3204 O H stretching 1658 H O H stretching 1617 H O H stretching 1028 Nb=O double bond 915 Nb=O double bond 874 Nb=O double bond 677 Nb O Nb linkages – symmetric stretching
595 M. M. M. HARING AND A. M. McDONALD
~3204 and ~3422 cm 1 and is associated with (Table 4). The XRPD data for franconite in this O H bending. A similar FTIR spectrum is study are broadly similar to those of Jambor et al. observed for ternovite (Mg,Ca)Nb4O11·nH2O (1984); however, the highest intensity line in this (Subbotin et al., 1997), implying that the study occurs at a d spacing of ~10 A˚ rather than mineral has crystal-chemical properties similar ~11 A˚ as observed by Jambor et al. (1984). X-ray to those of franconite. powder diffraction data from desiccated franco- nite (Na2Nb4O11·3H2O) collected by Jambor et al. (1984) showed a strong line occurring at a X-ray crystallography and crystal-structure d spacing of ~10 A˚ , similar to franconite in the determination present study. Subsequent re-runs of the desic- X-ray powder diffraction data were collected cated franconite over a 2 y period resulted in using a 114.6 mm diameter Gandolfi camera, XRPD data showing a doublet at a d spacing of 0.3 mm collimator and Fe-filtered CoKa radiation ~10 11 A˚ due to a gradual rehydration of (l = 1.7902 A˚ ). Intensities were determined using franconite (Jambor et al., 1984). These differ- a scanned image of the powder pattern and ences in hydration state may explain why the normalized to the measured intensity of d = highest intensity line for franconite from this 9.968 A˚ (I = 100). The measured intensities were study has different d spacings from that given by compared to a pattern calculated using results Jambor et al. (1984). from the crystal-structure analysis and the To obtain a crystal for the next stage of the program CRYSCON (Dowty, 2002) to determine study, individual crystals were separated from a how much an hkl plane contributed to a reflection. coarse-grained, franconite-bearing globule. These Overall, there is a good agreement between the were examined optically and one of dimensions measured and calculated powder patterns 0.08 mm60.02 mm60.01 mm, exhibiting a
TABLE 4. Franconite X-ray powder diffraction data.
Imeas Icalc dmeas dcalc hkl Imeas Icalc dmeas dcalc hkl (A˚ ) (A˚ ) (A˚ ) (A˚ )
100 100 10.211 9.968 1 0 0 5 1 2.114 2.112 1 0 6¯ 3 7 6.261 6.246 0 0 2¯ 2 2.097 1 3 0 23 16 5.479 5.407 1 1 0 5 1 2.044 2.029 0 3 2¯ 21 6 5.050 4.984 2 0 0 9 3 2.007 2.002 5 0 2¯ 16 4.925 1 0 2 1 1.998 3 1 4 6 5 4.265 4.271 2 0 2¯ 1 1.994 5 0 0 15 11 3.989 3.941 2 1 0 5 5 1.985 1.971 1 0 6 2 3.914 2 1 1¯ 4 2 1.918 1.911 5 1 2¯ 7 3.911 1 1 2 2 2 1.903 1.904 5 1 0 3 2 3.693 3.619 2 1 1 6 2 1.779 1.776 3 3 2¯ 3 3.605 2 0 2 2 1 1.764 1.766 1 2 6¯ 6 6 3.560 3.556 2 1 2¯ 1 1.761 3 3 1 21 15 3.213 3.218 0 2 0 2 1 1.730 1.736 2 3 3 32 14 3.157 3.145 2 1 2 2 1.736 4 1 4 12 3.138 1 0 4¯ 5 1 1.708 1.713 1 3 4 13 3.123 0 0 4¯ 2 1.7 5 2 2¯ 9 6 3.097 3.116 0 2 1¯ 1 1.695 5 2 0 16 19 2.843 2.843 3 1 2¯ 3 3 1.683 1.692 4 1 6¯ 3 1 2.725 2.704 2 2 0 2 1.69 3 3 2 3 2 2.629 2.628 2 1 4¯ 2 1.681 1 2 6 6 1 2.578 2.57 2 2 2¯ 5 1 1.597 1.596 0 4 1¯ 2 4 2.520 2.524 3 1 2 2 1 1.567 1.569 2 0 8¯ 4 6 2.303 2.302 4 1 2¯ 1 1 1.527 1.525 2 1 8¯ 6 2 2.262 2.247 1 2 4¯ 2 2.241 0 2 4¯
596 CRYSTAL CHEMISTRY OF FRANCONITE
simple extinction and no evidence of twinning, The Na site and additional O sites were identified was selected. X-ray intensity data were collected from subsequent Fourier difference maps. on a Bruker D8 three-circle diffractometer Refinement of the site-occupancy factors (SOF) equipped with a rotating-anode generator, multi- indicated that all of the cation and anion sites layer optics incident beam path and an APEX-II were fully occupied (Table 6). Determination of CCD detector. X-ray diffraction data (28,653 which O sites were occupied by OH or H2O was reflections) were collected to 60º2y using 20 s per based on bond-valence calculations and electro- 0.3º frame and with a crystal-to-detector distance neutrality considerations (Table 7). During the of 5 cm. The unit-cell parameters for franconite, later stages of refinement, seven H sites were obtained by least-squares refinement of 19,947 located in the difference Fourier maps. These reflections (I >10sI), are a = 10.119(2), b= were inserted into the refinement with H O 6.436(1), c = 12.682(2) A˚ and b = 99.91(3)º distances being constrained to be close to 0.98 A˚ . (Table 5). The original unit cell [a = 22.22(1), b= Refinement of this model converged to R = 4.63% 12.857(5), c = 6.359(4) A˚ and b = 92.24(6)º] and wR2 = 11.95%. determined by Jambor et al. (1984) was obtained by electron diffraction and indexing of an XRPD Description of crystal structure pattern; as a result the weak reflections supporting the halving of the unit cell along the a axis may Cation polyhedra have been missed. An empirical absorption The crystal structure of franconite has one correction (SADABS; Sheldrick, 1997) was [5]-coordinated Na site bonded to four H2O applied and equivalent reflections merged to groups and a single O atom, producing an give 28,634 unique reflections covering the NaO(H2O)4 polyhedron. This is unusual in rock- entire Ewald sphere. forming minerals, in that Na typically occurs in Solution and refinement of the crystal structure [6] or higher. However, in rare instances it can of franconite were performed using SHELXL-97 occur in [5] as in zeravshanite [Cs4Na2Zr3 (Sheldrick, 1997). The crystal structure was (Si18O45)(H2O)2; Uvarova et al., 2004] and solved using direct methods, using the scattering mejillonesite [NaMg2(PO3OH)(PO4)(OH)·H5O2; curves of Cromer and Mann (1968) and the Atencio et al., 2012] and even as low as [4], as scattering factors of Cromer and Liberman in vuonnemite [(Na11TiNb2 (Si2 O 7 ) 2 (1970). Phasing of a set of normalized structure (PO4)2O3(F,OH); Ercit et al., 1998)]. Average factors gave a mean value of |E2 1| of 0.912, Na O bond lengths in zeravshanite and mejillo- consistent with a centre of symmetry being nesite are 2.400 and 2.396 A˚ , respectively, with present {|E2 1| = 0.968 for centrosymmetric bond lengths for both minerals ranging from and |E2 1| = 0.736 for non-centrosymmetric}. 2.275(5) to 2.615(4) A˚ . In franconite, Based on this and the space-group choices Na (O,H2O) bond lengths range from ˚ available, P21/c (#14) was chosen as the correct 2.337(6) to 2.422(7) A with an average bond space group. Phase-normalized structure factors length of 2.390 A˚ (Table 8), in good agreement were used to give a Fourier difference map from with those observed for [5]Na in zeravshanite and which two Nb and several O sites were located. mejillonesite.
TABLE 5. Miscellaneous data for franconite. a (A˚ ) 10.119(2) Monochromator Graphite b (A˚ ) 6.436(1) Intensity-data collection y:2y c (A˚ ) 12.682(2) Criterion for observed reflections F>4s(F) b (º) 99.91(3) GooF 1.022 V (A˚ 3) 813.6 Total No. of reflections 28653 Space Group P21/c (#14) No. Unique reflections 2380 Z 4 R (merge %) 5.93 Radiation MoKa (50 kV, 40 mA) R % 4.63 wR2 % 11.95
597 2 TABLE 6. Atomic positions and displacement parameters (A˚ ) for franconite.
11 22 33 23 13 12 Atom Nb(2) 0.00108(5) 0.18674(7) 0.60733(4) 0.0228(3) 0.0057(2) 0.0076(2) 0.0006(2) 0.0045(2) 0.0001(2) 0.0119(1) McDONALD M. A. AND HARING M. M. M. O(1) 0.1710(4) 0.2516(6) 0.5586(3) 0.021(2) 0.011(2) 0.012(2) 0.001(1) 0.003(2) 0.000(2) 0.0147(8) O(2) 0.1673(4) 0.0287(7) 0.6170(3) 0.025(2) 0.011(2) 0.014(2) 0.000(1) 0.007(2) 0.001(2) 0.0160(8) O(3) 0.0729(5) 0.1963(7) 0.7489(3) 0.035(3) 0.014(2) 0.009(2) 0.000(1) 0.004(2) 0.001(2) 0.0191(9) O(4) 0.0845(5) 0.4391(6) 0.5862(3) 0.032(2) 0.006(2) 0.016(2) 0.000(1) 0.007(2) 0.003(2) 0.0179(9) O(5) 0.3290(5) 0.3395(8) 0.3959(4) 0.024(2) 0.022(2) 0.022(2) 0.003(2) 0.010(2) 0.001(2) 0.0223(9) 598 OH(6) 0.0483(4) 0.1383(6) 0.5714(3) 0.021(2) 0.008(2) 0.011(2) 0.003(1) 0.002(1) 0.001(1) 0.0134(8) OW(7) 0.7421(6) 0.3854(8) 0.3178(4) 0.035(3) 0.026(3) 0.024(2) 0.001(2) 0.009(2) 0.008(2) 0.028(1) OW(8) 0.6084(6) 0.2536(9) 0.5286(5) 0.031(3) 0.030(3) 0.031(3) 0.001(2) 0.001(2) 0.002(2) 0.031(1) OW(9) 0.4335(6) 0.378(1) 0.1641(5) 0.035(3) 0.037(3) 0.037(3) 0.003(3) 0.002(2) 0.006(2) 0.037(1) H(1) 0.124(5) 0.20(1) 0.619(5) 0.01(2) H(2) 0.78(1) 0.50(1) 0.364(8) 0.07(4) H(3) 0.78(1) 0.41(2) 0.254(5) 0.07(4) H(4) 0.60(1) 0.395(7) 0.553(9) 0.06(3) H(5) 0.68(1) 0.17(2) 0.57(1) 0.11(6) H(6) 0.347(4) 0.32(1) 0.133(6) 0.02(2) H(7) 0.511(8) 0.37(2) 0.128(9) 0.08(4) CRYSTAL CHEMISTRY OF FRANCONITE TABLE 7. Bond-valence table (v.u.) for franconite. Na Nb(1) Nb(2) S H(1) H(2) H(3) H(4) H(5) H(6) H(7) S O(1) 0.806;? 0.893;? 1.699 0.15;? 0.15;? 1.999 O(2) 0.812;? 0.857;? 1.669 0.15;? 0.17;? 1.989 O(3) 1.269;? 0.727;? 1.996 1.996 O(4) 1.251;? 0.739;? 1.990 1.990 O(5) 0.233;? 1.558;? 1.791 0.15;? 1.941 OH(6) 0.832;? 0.279;? 1.111 0.84;? 1.951 OW(7) 0.198;? 0.198 0.16;? 0.85;? 0.85;? 2.058 OW(8) 0.198;? 0.198 0.85;? 0.83;? 0.15;? 2.028 OW(9) 0.380;? 0.380 0.85;? 0.85;? 2.080 S 1.009 4.970 5.053 1.00 1.00 1.00 1.00 1.00 1.00 1.00 * Bond valences for H sites determined using parameters from Brown and Altermatt (1985). Bond valences for other sites were determined using parameters from Brese and O’Keeffe (1991). TABLE 8. Interatomic distances (A˚ ) in franconite. NaO(H2O)4 Polyhedron Nb(2)O4(OH)2 Octahedron Na O5 2.337(6) Nb2 O3 1.823(4) OW8 2.396(7) O4 1.829(4) OW7 2.398(6) O2 1.988(4) OW9 2.399(7) O1 1.990(4) OW9 2.422(7) OH6 2.217(4) Nb(1)O5(OH) Octahedron Nb1 O5 1.748(5) O1 1.956(4) O2 1.969(4) O4 2.023(4) O3 2.029(4) OH6 2.384(4) Hydrogen bonding Donor H d(D H; A˚ ) d(H_A; A˚ ) OH6 H1 0.980 1.868 164.48 2.824 OW7 OW7 H2 0.980 1.901 167.63 2.866 O1 OW7 H3 0.980 1.934 170.30 2.905 O2 OW8 H4 0.980 1.938 149.20 2.823 O5 OW8 H5 0.980 1.816 159.84 2.762 O2 OW9 H6 0.980 1.918 168.04 2.884 O1 OW9 H7 0.980 1.909 151.41 2.800 OW8 599 M. M. M. HARING AND A. M. McDONALD The crystal structure also contains two Nb sites centre displacement resulting in distorted octa- in octahedral coordination with O atoms and OH hedra with large ranges in bond length. These groups: Nb(1)O5(OH) and Nb(2)O4(OH)2 large ranges in Nb (O,OH) bond length are (Fig. 3). Both are highly distorted with consistent with those observed in the Nb(O,OH)6 Nb (O,OH) bond lengths ranging from present in the crystal structure of franconite. 1.748(5) 2.384(4) A˚ and 1.823(4) 2.256(4) A˚ , respectively (Table 8). The Nb(1)O5(OH) octahe- Bond topology dron has four equatorial bond lengths of ~2 A˚ as well as two asymmetrical apical bonds: a short The limited number of cation polyhedra present one of 1.746(5) A˚ and a long one of 2.383(4) A˚ , constrains the bond topology in franconite to the former possibly representing a Nb O double being relatively simple. First, the Nb(1)O5(OH) bond (Jehng and Wachs, 1990). This type of and Nb(2)O4(OH)2 octahedra are linked through octahedron is unusual in naturally occurring shared edges to form four-membered clusters minerals; however, similar octahedra are known consisting of two Nb(1)O5(OH) and two to occur in nenadkevichite (Rastsvetaeva et al., Nb(2)O4(OH)2 octahedra (Fig. 4). Each four- 1994), vuonnemite (Ercit et al., 1998), epistolite membered Nb(O,OH)6 cluster is subsequently (Sokolova and Hawthorne, 2004) and laurentia- linked to six adjacent clusters through shared nite (Haring et al., 2012). The Nb(2)O4(OH)2 corners, thus forming infinite sheets parallel to octahedron has three pairs of equivalent bonds: [100] (Fig. 4). This strongly affects the two short bonds with lengths of 1.823(4) and morphology of the crystals, which are platy and ˚ 1.829(4) A, two long bonds with lengths of flattened on {100}. The Na(O,H2O)5 polyhedra 2.217(4) and 2.256(4) A˚ and two bonds of are linked through shared corners to form single intermediate lengths of 1.988(4) and 1.990(4) A˚ . chains parallel to [100] (Fig. 5). These chains of This second type of octahedra is very rare in NaO(H2O)4 polyhedra form distinct layers which naturally occurring minerals and has only been alternate with layers of Nb(O,OH)6 octahedra reported in nacareniobsite-(Ce) [Na3Ca3REENb along [100]. Each NaO(H2O)4 polyhedral layer is (Si2O7)2OF3; Sokolova and Hawthorne, 2008]. In weakly linked to sheets of Nb(O,OH)6 octahedra both Nbj6 (j = O, OH, H2O) octahedra, the through shared O(5) anions and more importantly longest bond lengths correspond to Nb OH H bonds (Fig. 6), which are discussed below. bonds whereas the shortest bond lengths corre- spond to Nb O bonds, the short distances Hydrogen bonding probably reflecting the presence of double bonds. When in octahedral coordination, small The crystal structure of franconite is quite highly charged cations such as Nb5+ often cause interesting in light of the well defined significant O O repulsion which leads to strain H bonding that is essential to completing the on O M O bonds (Megaw, 1968a,b). To reduce crystal structure. There are three H2O groups this strain, the Nb5+ cation will undergo an off- [OW(7), OW(8) and OW(9)] and one OH [OH(6)] FIG. 3. The two distorted octahedral Nb sites in franconite. 600 CRYSTAL CHEMISTRY OF FRANCONITE FIG. 4. Octahedral layer in franconite consisting of Nb(1)O5(OH) octahedra (dark blue) and Nb(2)O4(OH)2 octahedra (orange). group present in the crystal structure. The H NaO(H2O)4 polyhedral layer and is located ˚ bonds associated with the OH and H2O groups are ~1.87 A from OW7 [coordinated with separated into two groups: (1) those within NaO(H2O)4 polyhedra], forming a H bond Na Nb polyhedral layers and (2) those between between the Nb(O,OH)6 octahedral layer and the the Na Nb polyhedral layers. The first group of NaO(H2O)4 polyhedral layer (Fig. 8a). The H bonds are restricted to the NaO(H2O)4 second set of H bonds involves H(2) and H(3) polyhedral layer. These involve H(4) and H(7) that are bonded to OW(7), both projecting that are bonded to the OW(8) and OW(9) groups, towards the Nb(O,OH)6 octahedral layer. Each respectively and project towards adjacent chains H(2) is located ~1.90 A˚ from O(1) atoms while ˚ of NaO(H2O)4 polyhedra (Fig. 7). Each H(4) is each H(3) is located ~1.93 A from O2, generating ˚ located ~1.94 A from O(5) atoms while each H(7) additional H bonds between the NaO(H2O)4 and ˚ is located ~1.91 A from OW8 groups generating Nb(O,OH)6 layers (Fig. 8b). The third set of H bonds between adjacent NaO(H2O)4 polyhedral interlayer H bonds involves H(5) and H(6) that chains within the NaO(H2O)4 polyhedral layer. are bonded to OW(8) and OW(9) groups, The second group, which includes three sets of respectively. Both H(5) and H(6) project from H bonds, occur between layers of NaO(H2O)4 the NaO(H2O)4 layers towards the Nb(O,OH)6 ˚ polyhedra and Nb(O,OH)6 octahedra. The bonds layers. Each H(5) is located ~1.82 A from O(2) in each set share similar orientations with respect atoms while H(6) is located ~1.92 A˚ from O(1), to one another. The first set involves H(1) that is both generating the remaining H bonds between bonded to OH(6) [coordinated with Nb(O,OH)6 the NaO(H2O)4 and Nb(O,OH)6 layers (Fig. 8c). octahedra]. Each H(1) projects towards the The second group of interlayer H bonds involve FIG. 5. Chains of edge-sharing NaO(H2O)4 polyhedra parallel to [100] 601 M. M. M. HARING AND A. M. McDONALD FIG. 6. The crystal structure of franconite projected onto [010] showing layers of NbO6 octahedra (dark blue) alternating with layers containing NaO(H2O)4 polyhedra (pink). H(1), H(2), H(3), H(5) and H(6) and these play a ferroelectric capacitors (Maso´ et al., 2011; Yim et critical role in the crystal structure of franconite. al., 2013). From a crystal-structure perspective, The predominance of H bonding serves to link the both Na2Nb4O11 and KCa2Nb3O10 are constructed Na(O,H2O)5 and Nb(O,OH)6 layers, thereby of alternating Nb polyhedral and (Na, K) completing the crystal structure of the mineral polyhedral layers similar to those in franconite, and also leads to the perfect [100] cleavage but with bond topologies that differ from those of observed in franconite. franconite. Synthetic Na2Nb4O11, like franconite, is monoclinic but it crystallizes in the space group C2/c. It has two unique Nb sites, but unlike those Related structures in franconite, one occurs as NbO7 bipyramids, The crystal structure of franconite is broadly with FIG. 7. The local environment of group 1 hydrogen bonds. These hydrogen bonds only occur within the Na(O,H2O)5 polyhedral layer and link together adjacent chains of NaO(H2O)4 polyhedra. 602 CRYSTAL CHEMISTRY OF FRANCONITE FIG. 8. The local environment of group 2 interlayer H bonds. These H bonds link together the Na(O,H2O)5 polyhedral layer and Nb(O,OH)6 octahedral layers. Group 1 H bonds occur in three sets including (a) Set1 involving H(1) atoms (b) Set 2 involving H(2) and H(3) atoms, and (c) Set 3 atoms involving H(5) and H(6) atoms. polyhedra (Maso´ et al., 2011). This differs from octahedra found in franconite. The Nb sites in the crystal structure of franconite where the KCa2Nb3O10 have Nb O bond lengths similar to Nb(O,OH)6 octahedra and NaO(H2O)5 polyhedra those observed in franconite. The first is similar to occupy distinct layers within the crystal structure. the Nb(2)O4(OH)2 octahedron in franconite, with The crystal structure of KCa2Nb3O10,unlike three pairs of equivalent bonds: two short bonds franconite, is orthorhombic with Cmcm space- both with lengths of 1.989(3) A˚ , two long bonds group symmetry. This compound is described as both with lengths of 2.024(4) A˚ and two bonds having a layered perovskite structure with distinct with an intermediate length of 1.905(7) A˚ layers of [6]Nb. There are two Nb sites in the (Fukoka et al., 2000). The second is similar to crystal structure, both being [6]-coordinated by O the Nb(1)O5(OH) octahedron in franconite, ˚ in NbO6 octahedra, rather than the Nb(O,OH)6 consisting of a short Nb O bond of 1.748(6) A, 603 M. M. M. HARING AND A. M. McDONALD ˚ a long Nb O bond of 2.389(7) A, as well as four NbO6 sites in SOMS, are similar to the equatorial bonds (Fukoka et al., 2000). Nb(1)O5(OH) sites in franconite, with a short ˚ ˚ Furthermore, the NbO6 octahedral layers in apical bond of ~1.8 A and a long one of ~2.4 A as KCa2Nb3O10 are the result of NbO6 octahedra well as four equatorial bonds with distances of ˚ linked to adjacent NbO6 octahedra through shared ~2 A (Nyman et al., 2002). corners. These layers of NbO6 layers contain Although their crystal structures have not yet interstitial Ca2+ cations and alternate with layers been determined, those of ternovite and hoche- of KO7 polyhedra (Fukoka et al., 2000). lagaite are expected to be related to that of SOMS, unlike Na2Nb4O11 and KCa2Nb3O10, franconite based on the similarity of their XRPD can be synthesized at low temperatures (T patterns and cationic ratios. This implies that, ~175ºC) under alkaline conditions (pH ~13.7; despite differences in chemistry amongst the Xu et al., 2004). Franconite is expected to have three minerals, the structure type that is common formed under similar conditions based on its to all three must be considered to be highly hydrous crystal structure (see discussion below) flexible, at least in terms of the alkalis and and association with carbonate minerals such as alkaline-earths that may be accommodated. siderite and calcite (this study; Jambor et al., Chemical data from franconite, hochelagaite 1984). SOMS exhibit a strong ion-exchange and ternovite (Fig. 9) indicate that a nearly selectivity for divalent cations over monovalent complete solid solution exists between franconite cations making them useful in removing heavy and hochelagaite; if one assumes similar metals such as Pb2+,Co2+ and Cd2+ from ground stoichiometries for the two, The Ca-Na substitu- water and soils (Nyman et al., 2001). tion presumably involves the generation of Nb-dominant SOMS [Na2Nb2O6·H2O; Nyman et vacancies or the conversion of O to OH (e.g. al., 2001] are hydrated and have crystal structures Ti4+ +OH-$ Nb5+ +O2– substitution in SOMS; with cation ratios similar to those of franconite. Nyman et al., 2001) in order to maintain charge- The crystal structure of Nb-dominant SOMS, like balance. In the crystal-structure refinement of franconite, is monoclinic but it crystallizes in the franconite, the O and OH sites were found to be space group C2/c (Nyman et al., 2001). It consists fully occupied. Furthermore, chemical data (n = of sheets of edge-sharing Na(O,H2O)6 octahedra 35) from five franconite-rich globules show a (Nyman et al., 2001) instead of chains of corner strong, inverse ~2:1 correlation between Na and sharing Na(O,H2O)5 polyhedra, as is observed in Ca in terms of atoms per formula unit (a.p.f.u.) franconite. These sheets of edge-sharing (Fig. 10), while similar trends involving substitu- Na(O,H2O)6 octahedra alternate with layers tions with other elements (e.g. Nb, Ti, Al and Si) containing double chains of NbO6 octahedra are not evident. As such, it is probable that the with Na(O,H2O)4 polyhedra (Nyman et al., Na-Ca substitution is a coupled substitution 2001) occurring between the double chains. The linked to the creation of vacancies, i.e. 2Na+ $ FIG.9.Na Ca Mg ternary system for franconite, hochelagaite and ternovite. 604 CRYSTAL CHEMISTRY OF FRANCONITE FIG. 10. Na vs. Ca content in the crystal structure of franconite. Values of Na and Ca are expressed as atoms per formula unit (a.p.f.u.) Ca2+ + &. From preliminary studies, it also more probable that the composition of the appears that the unidentified mineral UK56 (Chao individual phase that crystallizes is most et al., 1990) [(Ca0.5&0.5)Nb2O5(OH)·4H2O] has dependent on the composition of the fluids from a crystal structure and bond topology very similar which it is crystallizing. As such, the potential for to that of franconite. In this mineral, vacancies Fe2+-aswellasK+-rich compositions of due to Ca-Na substitution are observed where the franconite must be considered high. alkali/alkali-earth site is only half-occupied by The crystal structure of franconite is also highly Ca instead of being fully occupied by Na as in flexible with respect to water content. Electron franconite (unpub. data). An analysis of the microprobe data from franconite used in this study chemical data available for minerals of the indicate a calculated water content of 16.31 wt.%, franconite–hochelagaite–ternovite ternary compared to the water content of ~10 14 wt.% for system indicates limited substitution of Mg2+ franconite determined by Jambor et al. (1984). As and Ca2+. While the radii for [6]Ca2+ (1.00 A˚ )and water loss can occur under the vacuum conditions [6]Mg2+ (0.72 A˚ ) (Shannon, 1976) suggest that used for microprobe analysis, Jambor et al., (1984) any Ca $ Mg substitution should be limited on also measured the water content of franconite using an ionic-radius basis, the true extent of this a thermal analyzer. The results indicate that substitution is not known, primarily due to the franconite can accommodate from three H2O paucity of data related to the chemistry of groups under dry conditions to 26 H2O groups ternovite. Since the crystal structures of ternovite, under humid conditions (Jambor et al., 1984). This hochelagaite and franconite are expected to be wide variation in H2O content suggests that H2Oin related, the presence of ternovite containing both franconite can be either structurally bound or Ca2+ and Mg2+ further supports the flexibility of loosely bound, possibly adhering to the large the franconite structure. Given the difference in surface areas present in franconite, not unlike the ionic radii between [6]Na+ (1.02 A˚ )and[6]Mg2+ situation for many clays (Salles et al., 2009; (0.72 A˚ ), as well as the charge difference, Barshad, 1952). Results indicate that the loosely substitution between Na+ and Mg2+ would bound water is readily expelled under dry or likely be very limited. Although such a substitu- vacuum conditions, whereas part of the structurally tion is indicated on the franconite–hochelagaite– bound water is released upon heating to 110ºC with ternovite ternary, it is only supported by a single the remainder being released upon heating to data point representing a Na Mg Ca solid 356ºC (Jambor et al., 1984). The loss of the solution, thus the true extent of Na $ Mg structurally bound water in franconite leads to a substitution in franconite remains uncertain. collapse in the crystal structure, as evidenced by When considering the crystal-chemical flexibility the degradation of XRPD data collected on of the structure type that is common to materials heated at 150, 250, 350 and 500ºC franconite, hochelagaite and ternovite, it is (Jambor et al., 1984). The phases present in X-ray 605 M. M. M. HARING AND A. M. McDONALD patterns collected at 150, 250 and 350ºC could not Fa¨rber, G. (2012) Mejillonesite, a new acid sodium, be identified, but may correspond to magnesium phosphate mineral, from Mejillones Na2Nb4O11(H2O)n (n =1;Jamboret al., 1984). Antofagasta, Chile. American Mineralogist, 97, Upon heating samples of franconite to 500ºC, 19 25. X-ray patterns were found to be consistent with Babechuk, M.G. and Kamber, B.S. (2011) An estimate of 1.9 Ga mantle depletion using the high-field- Na2Nb4O11 (Jambor et al., 1984). Hydrogen bonds involving structurally bound water are essential in strength elements and Nd Pb isotopes of ocean floor stabilizing the crystal structure of franconite. The basalts, Flin Flon Belt, Canada. Precambrian structural collapse of the crystal structure of Research, 189, 114 139. franconite at T as low as 150ºC, can probably be Barshad, I. (1952) Adsorptive and swelling properties of ascribed to the loss of H bonds resulting from the clay-water system. Clays and Clay Minerals, 1, loss of structurally bound water. This structural 70 77. Belovitskaya, Y.V. and Pekov, I.V. (2004) Genetic collapse may possibly begin at temperatures as low mineralogy of the burbankite group. New Data on as 110ºC, as the release of structurally bound water Minerals, 39,50 64. starts at such T. Given the essential role of water in Brese, N.E. and O’Keefe, M. (1991) Bond-valence the crystal structure of franconite, it is likely that parameters for solids. Acta Crystallographica, B47, this mineral precipitated from aqueous fluids at low 192 197. temperatures, <110ºC. This is consistent with fluid Brown, I.D. and Altermatt, D. (1985) Bond-valence inclusion studies by Schilling et al. (2011) that parameters obtained from a systematic analysis of indicate the presence of late-stage fluids at Mont- the inorganic crystal structure database. Acta Saint Hilaire that were trapped at temperatures Crystallographica, B41, 244 247. lower than 340ºC. Chao, G.Y., Conlon, R.P. and Velthuizen, J. (1990) Mont Saint-Hilaire unknowns. The Mineralogical Acknowledgements Record, 21, 363 368. Cooper, M.A. and Hawthorne F.C. (2012) The crystal [4] 3+ The authors thank F.C. Hawthorne (Department structure of kraisslite, Zn3(Mn,Mg)25(Fe ,Al) 3+ 5+ of Geological Sciences, University of Manitoba) (As O3)2[(Si,As )O4]10(OH)16, from Sterling Hill for providing access to the four-circle diffract- mine, Ogdensburg, Sussex County, New Jersey, ometer and Mark. C. Cooper for providing USA. Mineralogical Magazine, 76, 2819 2836. assistance with the single-crystal XRD data Cromer, D.T. and Mann, J.B. (1968) X-ray scattering collection. They also thank F.C. Hawthorne and factors computed from numerical Hartree-Frock wave Mark C. Cooper for providing access to the functions. Acta Crystallographica, A24,321 324. electron microprobe and the University of Cromer, D.T. and Liberman, D. (1970) Relativistic Manitoba and Ravinder Sidhu for providing calculation of anomalous scattering factors for X rays. assistance with the chemical analysis. In addition Journal of Physics and Chemistry, 53,1891 1898. Dr Joy Gray-Munro (Department of Chemistry Dowty, E. (2002) CRYSCON for Windows and and Biochemistry, Laurentian University) is Macintosh Version 1.1. Shape Software Kingsport, thanked for providing access to the infrared Tennessee, USA. Ercit, T.S., Cooper, M.A. and Hawthorne, F.C. (1998) spectrometer as well as assistance with data The crystal structure of vuonnemite, collection. The authors also acknowledge the Na Ti4+Nb (Si O ) (PO ) O (F,OH), a phosphate- comments made by Peter Leverett and an 11 2 2 7 2 4 2 3 bearing sorosilicate of the lomonosovite group. The anonymous referee as well as those of the Canadian Mineralogist, 36, 1311 1320. editorial board member, Ed Grew. Financial Fielicke, A., Meijer, G. and Von Helden, G. 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